Abstract - Grazing and Intraspecific Variation in Four North American Grass Species

Although intraspecific variation in grasses has been the subject of many studies, few have examined grazing-related variation in native North American grasses and even fewer its relationships with native mammalian grazers. I examined intraspecific variation in Bouteloua gracilis, Agropyron smithii, Schizachyrium scoparium, and Andropogon gerardii, important components in the diets of bison, elk, pronghorn, and prairie dogs in Wind Cave Nation Park (WCNP), South Dakota.

Grazer impact was examined along a grazing gradient which included grazed young- and old-active-prairie-dog-colony, extinct-colony, and noncolony sites, as well as a grazing exclosure. Plants were more frequently and more heavily grazed on active-colony sites, where all the grazers preferentially feed.

Morphological data from plants growing in situ and in uniform environments were used to examine the relationships of intraspecific variation to grazing. Patterns of variation of plants in situ corresponded to the pattern of present grazer use, while patterns in plants in common environments were better explained by patterns of grazing history of the plant populations. Both in situ and in common environments, plants from heavily grazed sites were small and more prostrate than those from sites with little or no grazing. Differences among populations were more clinal than discrete, but extremes of the gradient were distinct and probably represent genetically based differentiation related to grazing history of the plants sites of origin in WCNP.

In separate experiments, A. smithii and B. gracilis from the grazing gradient extremes (prairie-dog-colony sites and the grazing exclosure) were grown in monocultures and interpopulation replacement-series mixtures, with and without defoliation. A complex set of responses to population, defoliation, density-independent competition was found. Responses differed between species. However, in both species, the grazing history of the sites of origin in WCNP influenced responses to both competition and defoliation; both morphology and biomass production differed between populations. Defoliation more negatively affected exclosure plants than colony plants, while competition more often adversely affected colony plants. Affects of competition usually disappeared with defoliation.